The present invention relates to a flux-cored wire for welding shielded by a flow of gas, to an MAG (Metal Active Gas) welding process, i.e. a gas-shielded welding process using such a meltable wire, and to a welded joint obtained by melting the said meltable
The flux-cored wires used for gas-shielded welding usually consist of a tubular outer metal sheath, also called a foil, and of a central core comprising filling elements, for example a metal powder.
Such flux-cored wires are used in many gas-shielded welding procedures, in particular in the welding of normalized structural steel, TMCP (Thermo Mechanical Control Process) steel and tempered-and-annealed steel.
At the present time there are three types of flux-cored wire which can be used for the gas-shielded welding of metal workpieces, especially of structural steel workpieces, namely xe2x80x9cbasicxe2x80x9d, xe2x80x9crutilexe2x80x9d and xe2x80x9cslag-freexe2x80x9d wires, also called xe2x80x9cmetal-coredxe2x80x9d wires. These various flux-cored wires are differentiated by the nature of the various elements of which they are composed.
Thus, depending on the type of flux-cored wire used in the welding process, there is a greater or lesser tendency for the flux-cored wire to undergo soft melting, arc stability may increase or decrease, positional welding may become easier or harder, the amount of molten metal spattered may increase or decrease, etc.
Likewise, depending on the type of flux-cored wire used in the welding process, and therefore depending on the nature and content of the various elements of which this flux-cored wire is composed, the metallurgical properties of a weld obtained, i.e. of a metal deposited, will vary greatly.
Thus, it is known that the characteristics of the weld depend on the content of the metal deposited in terms of its various elements such as, in particular, the elements: oxygen, titanium, niobium, vanadium, hydrogen, etc.
By way of example, Table I below indicates, on the one hand, the main elements involved in the constitution of the 3 aforementioned different types of flux-cored wires and, on the other hand, for each of them, the operational and compositional consequences which conventionally result therefrom.
It is apparent from the above Table I that the flux-cored wires of the xe2x80x9crutilexe2x80x9d type, based on titanium oxide (TiO2), have better operating properties than the other types of wire: very good arc stability, a regime of metal transfer by axial spraying for a wide range of parameters resulting in the almost complete absence of spatter during welding, slag with a high melting point allowing a high deposition rate in positional welding, etc.
However, from the composition of its slag, i.e. of the filling elements of which its central core is composed, the xe2x80x9crutilexe2x80x9d-type flux-cored wire has the drawback of leading to a weld containing residual elements (oxygen, niobium, vanadium, etc.) which tend to prevent optimum mechanical properties of the deposited metal, i.e. the weld, being achieved.
More specifically, the oxygen content of the deposited metal, although this may vary depending on the nature and quantity of deoxidizing elements present in the filling powder, cannot, in the case of xe2x80x9crutilexe2x80x9d-type wires, be reduced to as low a level as, for example, in the case of xe2x80x9cbasicxe2x80x9d-type wires.
It follows that a weld produced using a xe2x80x9crutilexe2x80x9d-type wire has a lower ductile fracture energy due to a higher amount of inclusion.
Likewise, since the slag of the xe2x80x9crutilexe2x80x9d-type wire is mostly composed of rutile, i.e. of titanium oxide (TiO2), titanium is inevitably found in the deposited metal and in a quantity which varies depending, in particular, on the oxido-reduction reactions occurring in the arc and on the metal-slag exchanges taking place within the weld pool, i.e. within the molten metal.
The titanium content of the deposited metal cannot therefore be adjusted as required, but depends closely on all the chemical elements present, which must be balanced depending on the mechanical properties (tensile strength, yield stress, etc.) which the weld must have, which themselves depend on the type of steel that has to be welded.
Furthermore, with regard to the niobium and vanadium contents of the deposited metal, these also cannot be reduced below a certain threshold, given that these elements exist as impurities in the natural titanium oxides normally used for manufacturing welding products.
The use of synthetic and therefore relatively pure, titanium oxide partly solves this problem, but to the detriment of a significant increase in the cost of the flux-cored wire and at the cost of greater complexity of the process for manufacturing the latter.
Extensive research carried out with the aim of improving the metallurgical properties of xe2x80x9crutilexe2x80x9d-type flux-cored wires have resulted in flux-cored wires exhibiting a xe2x80x9ctitanium-boronxe2x80x9d effect.
Specifically, it has been shown that the presence of a very small quantity of boron, in general 20 to 60 ppm, in the deposited metal considerably retards the appearance of proeutectoid ferrite at the austenitic grain boundaries as the weld deposit cools down.
Thus, transformation of the austenite can occur by intragranular nucleation of ferrite on the fine inclusions, which are always present in a weld, provided that the latter contain a certain quantity of titanium.
A structure having very fine ferrite grains is usually called xe2x80x9cacicular ferritexe2x80x9d; it has improved tensile properties (tensile strength, yield stress, etc.) and toughness (Charpy V-notched impact strength, CTOD, etc.).
In other words, during cooling of the weld, the titanium-boron effect prevents the formation of coarse proeutectoid ferrite, which is very prejudicial to the toughness of the weld, and finally produces a structure of the acicular-ferrite type, consequently making the welds produced using wires of the rutile type (but also of the slag-free type) compatible, in the as-welded state, with the most stringent industrial requirements, which could not be met by wires of the basic type.
However, although the results obtained are satisfactory in the as-welded state, the same does not apply when the weld undergoes a postwelding heat treatment necessary, in particular, for stress relieving the weldment, especially in the case of joining very thick workpieces.
This is because it has been observed that a weld produced by means of a xe2x80x9ctitanium-boronxe2x80x9d rutile wire, which was subjected to a subsequent heat treatment, lost its metallurgical properties and therefore the benefit provided by the xe2x80x9ctitanium-boronxe2x80x9d
Consequently, several publications report tests relating to the formulation of filling powders for flux-cored wires, in particular of xe2x80x9crutilexe2x80x9d type, which were intended to improve the behaviour of the welds after a postwelding heat treatment.
Furthermore, among the various elements which may be found in flux-cored wires, mention should be made of nitrogen.
Thus, document JP-A-63,220,996 teaches a flux-cored wire whose sheath contains 220 ppm nitrogen.
Furthermore, documents JP-A-63,278,697 describes a flux-cored wire containing 1-12% metal fluoride, 0.1-1.5% metal carbonate, 0.2-3% of one or more mineral silicates and silica chlorides, 0.2-2% silica, 1-2.5% manganese, 0.05-0.3% titanium and 0.1-1% magnesium; the weight of the filling flux representing from 10 to 30% of the total weight of the flux-cored wire.
Moreover, the document I.S.I.J. INTERNATIONAL, Vol. 35, 1995, VO 10 teaches, moreover, that the low-temperature impact strength of the weld is improved by increasing the nitrogen content up to levels reaching the preferred values of about 80 ppm.
However, at the present time none of the existing flux-cored wires, in particular of the rutile type, produces a weld having good toughness and impact-strength properties after a postwelding heat treatment.
One of the aims of the present invention is therefore to solve the abovementioned problem and to provide a flux-cored wire, in particular of the rutile or slag-free type, for gas-shielded welding, especially for structural steels, such as normalized, TMCP or tempered-and-annealed steels, making it possible to obtain a weld having excellent low-temperature toughness and impact-strength properties, both in the as-welded state and after a stress-relieving heat treatment.
Another objective is to provide a flux-cored wire making it possible to obtain a reliable and homogeneous weld, i.e. having the same properties in all the zones of the molten metal of the weld, in particular in the case of a multiple-pass weld.
In general, the properties of a weld depend on the various elements of which the weld is composed and on their respective relative proportions, given that these elements may have, depending on the case, intrinsic actions which are independent of each other or actions which are combined with each other, especially of the synergistic type or, conversely, of the antagonistic type.
Thus, in most cases, the presence of a particular element in the wire and/or the deposited metal has a positive influence on certain mechanical properties of the weld which results therefrom and a negative influence on other properties.
Consequently, to find a suitable formulation for a flux-cored wire is not an easy thing to do when it is desired to balance the composition of the welded joint, i.e. the deposited metal, in order to obtain a weld having optimum mechanical properties for a given application, such as the strength and toughness.
Furthermore, it is also necessary, or even indispensable, also to take into account the operating parameters. This is because, it may be readily understood that a flux-cored wire achieving a good metallurgical compromise is of no use if the formulation, i.e. its composition, used for manufacturing it makes it very difficult to use from an operating standpoint.
Thus, basic-type flux-cored wires are reputed to have a greatly superior metallurgical potential to rutile-type flux-cored wires but, on the other hand, have operating characteristics which are greatly inferior to those of the rutile wires, especially in terms of arc stability, amount of spatter, productivity in positional welding, etc.
This has then resulted in a gradual disappearance of this type of flux-cored wire on the market as the metallurgical properties of rutile flux-cored wires advance and improve.
Consequently, there is a real need for flux-cored wires which are acceptable not only from the standpoint of their metallurgical properties but also from the standpoint of their operating properties, this problem having hitherto not been solved or only partially solved.
Faced with this problem, the inventors of the present invention have demonstrated that the total content of the flux-cored wire in terms of certain base elements, especially nitrogen, of the deposited metal surprisingly played an essential role in the quality and properties of the weld, especially in its toughness and its impact strength.
More specifically, the inventors have discovered that, in order to obtain a quality weld, in particular after a treatment for stress relieving the weldment thus obtained, it is necessary to have, on the one hand, a low nitrogen content in the deposited metal and, on the other hand, a defined content in terms of the other constituents likely to be found therein such as, especially, aluminum, boron, niobium, vanadium, manganese, titanium, carbon, nickel, chromium and, in particular, aluminum, and titanium, etc.
The present invention therefore relates to a flux-cored wire for gas-shielded welding, preferably MAG welding, composed of at least one outer metal sheath and at least one central core comprising filling elements, characterized in that it contains, with respect to the total weight of the flux-cored wire:
less than 75 ppm nitrogen, preferably less than 60 ppm nitrogen and advantageously less than 50 ppm nitrogen,
from 0.1 to 1% silicon, preferably from 0.2% to 0.9% silicon and advantageously from 0.25% to 0.75% silicon,
from 1% to 2.5% manganese, preferably from 1.2% to 2% manganese and advantageously from 1.4% to 1.8% manganese,
less than 7% titanium,
from 1 ppm to 110 ppm boron, preferably from 20 ppm to 90 ppm boron and preferably from 25 ppm to 70 ppm or 80 ppm boron,
less than 0.3% aluminum, preferably from 1 ppm to 0.2% aluminum, and advantageously from 5 ppm to 0.1% aluminum.
from 10 ppm to 200 ppm niobium, preferably from 10 ppm to 130 ppm niobium and preferably from 10 ppm to 100 ppm niobium, and
from 10 ppm to 200 ppm vanadium, preferably from 10 ppm to 130 ppm vanadium and preferably from 10 ppm to 100 ppm vanadium.
Within the context of the invention, it will be assumed that the total content of a given element in a flux-cored wire, for example nitrogen, is equal to the sum of the contents of the given element, for example nitrogen, in the various constituents of the flux-cored wire, namely, on the one hand, the sheath, and, on the other hand, the various filling elements.
Thus, for example, in the case of nitrogen the total content of nitrogen (N2) may be determined by applying the following formula (I):                                           [                          N              2                        ]                    t                =                                            (                              1                -                t                            )                        ·                          [                              N                2                            ]                                +                                    ∑                              i                =                1                                            i                =                n                                      ⁢                                                            [                                      N                    2                                    ]                                i                            ·                              [                I                ]                            ·              t                                                          (        I        )            
in which:
[N2]T represents the total nitrogen content (ppm by weight) of the flux-cored wire;
t represents the filling fraction of the flux-cored wire and is given by the following formula (II):                     t        =                              Weight of the filling elements                                              Filling elements weight                        +                          Sheath weight                                                          (        II        )            
[N2]s represents the nitrogen content (ppm by weight) of the metal sheath of the flux-cored wire;
n represents the number of individual substances i making up the filling elements and likely to contain nitrogen;
[N2]i represents the nitrogen content (ppm by weight) of each individual substance i; and
[i] represents the proportion of each substance i with respect to the total weight of filling elements.
Of course, the respective total contents of each of the other elements of which the flux-cored wire is composed may be determined in a similar manner.
Depending on the case, the flux-cored wire according to the invention may contain:
from 300 ppm to 0.14% carbon (excluding carbonates), preferably from 0.03 to 0.1% carbon and preferably from 0.03% to 0.08% carbon, and/or
from 0.2% to 1% magnesium and/or zirconium, preferably from 0.3% to 0.9% magnesium and/or zirconium (Zr) and preferably from 0.4% to 0.8% magnesium and/or zirconium, and/or
from 0.01% to 0.35% sodium, potassium and/or lithium (the sum of the contents of the elements Li, Na and K possibly present is considered here), preferably from 0.03% to 0.3% sodium (Na), potassium (K) and/or lithium (Li) and preferably from 0.04% to 0.25% sodium, potassium and/or lithium, and/or
from 0.03% to 10% fluorine (F), this having a favourable effect on the diffusible hydrogen and/or
less than 0.02% sulphur, phosphorus, tin, antimony and/or arsenic (the respective contents of the elements S, P, Sn, Sb and As possibly present are considered here, and not the sum of the contents of the
various elements), preferably from 1 ppm to 0.012% sulphur, phosphorus, tin, antimony and/or arsenic and preferably from 10 ppm to 0.009%.
Furthermore, the flux-cored wire of the invention has a Bruscato coefficient (X) such that:
X=0.1xc3x97[P]+0.05xc3x97[Sb]+0.04xc3x97[As]+0.01xc3x97[Sn]
with X less than 20 ppm, preferably X less than 15 ppm and advantageously X less than 12 ppm,
where [P], [Sb], [As] and [Sn] are the total amounts (in ppm) of phosphorus, antimony, arsenic and tin, respectively, in the flux-cored wire.
In general, the outer sheath represents at least 50% of the total weight of the flux-cored wire, preferably from 65 to 90% of the total weight of the flux-cored wire.
Moreover, the diameter of the flux-cored wire is normally between 0.8 mm and 2.4 mm, preferably between 1 and 2 mm and advantageously between about 1.20 and about 1.60,
Depending on the embodiment chosen or the subsequent application in question, the flux-cored wire is preferably of the rutile (TiO2) type or of the xe2x80x9cslag-freexe2x80x9d type.
For a rutile-type wire, the Tio2 content of the wire may vary between 3 and 12%, which corresponds to a titanium content of 2% and 7%, approximately. Furthermore, the fluorine content of the rutile flux-cored wires may vary between 0.03% and 0.15%, preferably between 0.05% and 0.09%.
For a wire of the xe2x80x9cslag-freexe2x80x9d type or for a wire of the xe2x80x9cbasicxe2x80x9d type, the titanium content is 80 ppm to 2000 ppm, approximately, preferably from 100 to 1000 ppm and advantageously from 110 to 500 ppm.
On the other hand, the fluorine content of xe2x80x9cslag-freexe2x80x9d flux-cored wires may vary between 0.02% and 0.15%, preferably between 0.035% and 0.09%, while for a xe2x80x9cbasicxe2x80x9d-type wire the fluorine content may vary between 1.6 to 10%, because of the presence of fluorspar (CaF2)
The present invention also relates to a MAG gas-shielded welding process, in which a welded joint is produced by melting at least part of a flux-cored wire according to the inventionxe2x80x94preferably the wire is melted by supplying a welding energy of between 6 and 40 kJ.cmxe2x88x921.
From that, the invention also relates to such a welded joint which can be obtained by the process of the invention, especially a welded joint or deposited metal containing, with respect to the total weight of the welded joint, less than 80 ppm nitrogen, from 0.02% to 0.07% oxygen, from 0.02% to 0.12% carbon, from 0.15% to 0.6% silicon, from 1% to 1.9% manganese, less than 0.02% sulphur, less than 0.02% phosphorus, from 0.0025% to 0.08% titanium, from 0.0002% to 0.008% boron, less than 0.03% aluminum, preferably from 5 ppm to 150 ppm aluminum, from 10 ppm to 0.02% niobium and from 10 ppm to 0.02% vanadium.
Carbon (C) is one of the base elements of steels which affects the tensile and hardness properties of the various microstructures. Thus, to obtain a welded joint having a high tensile strength, it is desirable for the amount of carbon in the welded joint to have a relatively high value. However, it should not exceed a maximum value of 0.12% by weight as, above this, the risks of hot and cold cracking greatly increase. Moreover, given that carbon also acts as a deoxidizing agent, it is necessary to comply with a minimum carbon content of at least 0.02% by weight in the welded joint so as to allow effective removal of part of the oxygen and thus to obtain a welded joint with the correct toughness.
Silicon (Si), like carbon, is a deoxidizing agent which makes it possible to ensure good toughness of the welded joint and, moreover, has beneficial effects on the wetting of weld beads above a minimum concentration by weight of 0.15%. However, silicon also has a highly pronounced hardening effect in the various microstructures and increases the amount of martensitic and austenitic constituents of the welded joint. These have deleterious effects on the toughness of the welded joint. It follows therefore that it is necessary to keep the maximum silicon content at 0.6% by weight.
Manganese (Mn) is also one of the base elements of steels. In order to allow the appearance of a substantial quantity of acicular ferrite in the zones with the as-solidified structure and to improve their toughness, it is necessary to respect a minimum content of 0.9% by weight of manganese in the welded joint. Above this minimum content value of 0.9%, manganese plays a fundamental role in adjusting the hardenability of the molten metal to thermal welding cycles and, thus, in adjusting the quantity of acicular ferrite formed during the process of cooling the welded joint depending on the welding energy which has to be used in the envisaged application. However, it is necessary to maintain a manganese content below 1.9% by weight given that, above this figure, a deleterious hardening effect outweighs its beneficial effect of refining the microstructure of the welded joint and the toughness then generally deteriorates.
As regards sulphur (S), this considerably increases the risks of hot cracking of the welded joint when its content in the deposited metal is greater than 0.020%. Moreover, sulphur also has a deleterious effect on the ductility and toughness properties of the weld. Consequently, it is preferred to keep sulphur at a maximum concentration of at most 0.010%.
Phosphorus (P) has, like sulphur, but to a lesser extent, a negative effect on hot cracking of the welded joint when its content in the deposited metal is greater than 0.020%. Furthermore, during the cooling process following a postwelding heat treatment, the presence of phosphorus increases the brittleness of the weld zones which have maintained an as-solidified structure. Consequently, it is preferred, again, to keep phosphorus at a maximum concentration of at most 0.010%.
Titanium (Ti) must be present at a minimum content of 0.0025% by weight in order to ensure nucleation of acicular ferrite, which is a microconstituent indispensable for obtaining the correct toughness in the weld zones having an as-solidified structure. Above 0.080% by weight, titanium has, on the other hand, a tendency to contribute to the formation of a bainitic structure which impairs the toughness.
Boron (B) slows down the kinetics of formation of proeutectoid ferrite at austenitic grain boundaries during cooling of the weld beads. Consequently, boron makes it possible, in the presence of titanium, to increase the amount of acicular ferrite and therefore to improve the toughness of the weld above a total content of 0.0003% (3 ppm) by weight and this is so up to a content of 0.008% by weight (80 ppm). However, above 0.008%, boron increases the risks of hot cracking, which is not acceptable. In general, a boron content of 10 ppm to 70 ppm, and preferably between 20 ppm and 60 ppm, is maintained.
Niobium (Nb) and vanadium (V) contribute, in the presence of titanium, to increasing the amount of acicular ferrite and therefore the toughness characteristics of the welded joint for a minimum value of 0.001% by weight. However, these components also have a great affinity for nitrogen and carbon, so that they will precipitate in the form of carbides, nitrides or carbonitrides in the reheated zones of the welded joint during the execution of successive welding passes, as well as in all the zones during a postwelding heat treatment, and in this case with even greater intensity. it is therefore absolutely essential to limit their respective contents to at most 0.02% and preferably at most 0.01%.
As explained above, nitrogen (N) has, in solid solution or in the form of precipitates, an embrittling effect in welded joint in normalized structural, TMCP or tempered-and-annealed steels. Consequently, its total content in the deposited metal is maintained at a value of at most 80 ppm, preferably at most 60 ppm and advantageously at most 50 ppm.
In steel sheet, aluminum (Al) is often added, on the one hand, for the purpose of killing, i.e. fixing the oxygen in the form of aluminates and, on the other hand, so as to minimize the deleterious effect of nitrogen by trapping it in the form of nitrides. Consequently, care is usually taken to adjust the aluminum content in the said sheets such that the ratio of the contents, Al/N, is greater than 4. However, the same does not apply in welds of these steels. This is because it proves to be the case that producing a welded joint with such a ratio, Al/N greater than 4, leads to a catastrophic degradation in the toughness characteristics of the welded joint as having such a ratio of contents inevitably results in at least partial suppression of the formation of acicular ferrite and to an excessive increase in the titanium content in welded joints produced using flux-cored wires of the rutile (TiO2) type, thus causing a further decrease in the formation of acicular ferrite advantageously beneficial to the formation of undesirable bainitic-type microconstituents. It therefore follows that the total aluminum content must be kept at a value of less than 0.030% by weight, preferably less than 0.015% by weight and preferably less than 0.010% by weight. Furthermore, it is desirable to have a ratio of contents Al/N less than 4, preferably Al/N less than 3, more preferably Al/N less than 2 and advantageously Al/N less than 1.5.
Steelmakers generally try to decrease as far as possible the oxygen (O2) content of steels since oxygen tends to reduce the toughness and ductility properties of steels because of the formation of inclusions within them. Contrary to this, an oxygen content in welds of at least 0.02% by weight proves to be indispensable for allowing, in association with titanium, the formation of fine dispersed inclusions acting as nuclei or initiators for acicular ferrite and thus for obtaining good toughness in the weld areas having an as-solidified structure. However, it is necessary to respect a maximum oxygen content of about 0.07%, preferably a maximum content of 0.06%, or even 0.055%, in order to avoid a significant decrease in the ductile fracture energy in the welded joint, as is known in the case of steels.
Within the context of the present invention, it is considered that the nitrogen content of the deposited metal is substantially equal to that of the flux-cored wire used for producing the weld. However, when the gas shield employed during the welding operation is imperfect, a slight contamination of the deposited metal by atmospheric nitrogen is sometimes observed. Nevertheless, in the present case, it is considered that the gas shield is almost perfect and therefore that such a contamination of the deposited metal by atmospheric nitrogen does not occur. In other words, for the sake of simplifying matters, it is considered that the total amount of nitrogen present in the weld, i.e. in the deposited metal, essentially comes from the flux-cored wire used for producing the weld.
Furthermore, the welded joint according to the invention may also include up to 3.5% nickel, up to 0.5% chromium, up to 0.7% molybdenum and/or up to 0.6% copper.
This is because nickel (Ni) increases, at least slightly, the hardness and tensile properties of the various microstructures involved, in solid solution, in the iron lattice. It should be noted that this favourable effect of nickel is not accompanied by a deterioration in the ductility and toughness of these microstructures. However, it is preferred to keep the nickel content below 3.5% by weight in order to avoid hot cracking of the welded joint.
Furthermore, molybdenum (Mo) is an element which improves the hardenability of steels. Its ability to cause hardening by a solid-solution effect is also appreciable. Thus, its presence in the welded joint is recommended for applications involving a high welding energy, for example from 20 to 50 kJ/cm, requiring good hot behaviour of the steel, for example at 250xc2x0 or 350xc2x0 C., or when the assembly involves a steel having a high or very high elasticity, for example 500 to 900 MPa. However, it is preferred to keep the molybdenum content below 0.7% by weight in order to avoid decreasing the toughness of the welded joint.
Chromium (Cr), like molybdenum, may be added to improve the tensile and hardness properties of the welded joint, in particular for steels having a high or very high elasticity. However, it is preferred to keep the chromium content below 0.5% by weight, or even below 0.3% by weight, in order to avoid, again, decreasing the toughness of the welded joint.
Copper (Cu) may be added to the molten metal in order to improve the resistance to atmospheric corrosion, in amounts up to about 0.6% by weight.
Moreover, the welded joint may include phosphorus, antimony, arsenic and/or tin and have a Bruscato coefficient (X) such that:
X=0.1xc3x97[P]+0.05xc3x97[Sb]+0.04xc3x97[As]+0.01xc3x97[Sn]
with X less than 20 ppm and preferably X less than 15 ppm,
where [P], [Sb], [As] and [Sn] are the total amounts (in ppm) of phosphorus, antimony, arsenic and tin, respectively, in the said welded joint.
The invention furthermore relates to a steel component having a welded joint as described above.
Advantageously, the flux-cored wire of the invention is used for the construction of installations associated with the oil industry, such as an offshore oil platform a storage tank, or for shipbuilding.